How to set Import Units when importing a collada file with python script?
WM_OT_collada_import is the bl_idname of the Collada Import Operator in the C-code notation (in Python it's wm.collada_import, but the importer is a C operator).

import_units is an operator property.

If you want to set this property to True in an operator call, give it a keyword-argument:

bpy.ops.wm.collada_import(filepath=..., import_units=True)